Calling a module function in a ruby module

Question

I want to call a module function to define a constant in a utility module in ruby. However, when I try this I get an error message. Here comes the code and the error:

module M
  ABC = fun

  module_function

  def self.fun
    "works"
  end
end

Error message:

NameError: undefined local variable or method `fun' for M:Module

Any ideas? I also tried it without self and with M.fun but no success...

So you want to set the value for constant ABC using the result of method fun, isn't it? — Edvaldo Silva de Almeida Jr
– Edvaldo Silva de Almeida Jr, Commented Mar 3, 2017 at 22:20
correct. I want it computed once and never changed again. So ideally, I would put a freeze on it, but that would complicate the current question. — grackkle
– grackkle, Commented Mar 3, 2017 at 22:23

spickermann · Accepted Answer · 2017-03-03 22:51:18Z

8

It is just that the method is not defined when you assign fun to ABC. Just change the order:

module M
  def self.fun
    "works"
  end

  ABC = fun
end

M::ABC
#=> "works"

If you dislike the order (constants below methods), you might want to consider to have the method itself to memorize its return value. A common pattern looks like:

module M
  def self.fun
    @cached_fun ||= begin
      sleep 4     # complex calculation
      Time.now    # return value
    end
  end
end

M.fun
# returns after 4 seconds => 2017-03-03 23:48:57 +0100
M.fun
# returns immediately => 2017-03-03 23:48:57 +0100

edited Mar 3, 2017 at 22:51

answered Mar 3, 2017 at 22:25

spickermann

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5 Comments

grackkle Over a year ago

wow, that is weird. I don't need to define private methods before I use them in a class. So why do I need to do it in a module?

spickermann Over a year ago

Because constants are assigned when the Ruby files is loaded. Whereas methods call other methods at runtime, after the file was loaded.

grackkle Over a year ago

ok, so basically no way to define the constants first (which is the main information, that I want on top) and the methods later.

Jörg W Mittag Over a year ago

@bln-tom: Why is that weird? You call the method on line 2, at which point the method definition on line 6 hasn't even been parsed yet, let alone executed. How could Ruby possibly know what code to execute, if it hasn't even parsed the definition yet; it can't time-travel into the future? This is the same with private methods, by the way. If you call them before they are defined, that won't work either.

grackkle Over a year ago

@JörgWMittag To be more precise I meant private methods of classes. In this case, it is totally fine to first write public methods and then private methods. I even consider it a good practice, since the public interface of the class is what you see when you open the file.

Edvaldo Silva de Almeida Jr · Accepted Answer · 2017-03-03 22:28:28Z

2

Test this in you irb console:

$ irb 
2.3.3 :001 > module M
2.3.3 :002?>   def self.fun
2.3.3 :003?>     "worked"
2.3.3 :004?>   end
2.3.3 :005?>   
2.3.3 :006 >   ABC = fun
2.3.3 :007?> end
 => "worked" 
2.3.3 :008 > M
 => M 
2.3.3 :009 > M::ABC
 => "worked" 
2.3.3 :010 >

The fact is that now you defined self.fun before using it.

In your code you used the method before defining it.

answered Mar 3, 2017 at 22:28

Edvaldo Silva de Almeida Jr

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2 Comments

grackkle Over a year ago

basically the same answer. I voted for the faster one, sorry.

Edvaldo Silva de Almeida Jr Over a year ago

Sure! I was probably typing when he posted. But the important part is that your question was answered.

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